The present invention relates generally to well perforating, such as is practiced in the petroleum industry, and specifically to carriers for perforating guns holding shaped charges utilized in perforating well bore casing and producing formations.
Shaped charges are employed to perforate casing and surrounding producing formations due to their ability to produce long, tunnel-like perforations in a producing formation without the use of a projectile and without injecting a great deal of debris and residue into the perforations. However, the penetration characteristics of a shaped charge jet are greatly dependent upon the stand-off of the shaped charge, which may be defined as the distance between the base of the liner cone in a shaped charge and the nearest significant obstruction in front of the charge, which may be a cover over the mouth of the charge, the inner wall of a perforating gun carrier, or the inner portion of a plug in a gun port of a carrier, all of which are well known and widely employed in the art. In recent years, the petroleum industry trend has been toward the use of "high density" perforating, which involves the clustering of multiple shaped charges and the stacking of these clusters in the perforating gun carrier to effect 12 or more perforations per foot of interval of producing formation. Charge clusters are usually rotated with respect to the adjacent ones above and below them, to provide rotationally offset perforations which have a reduced tendency to weaken casing and are thought to provide better overall fluid flow from the producing formation. However, clustering of charges brings about a very significant decrease in the stand-off for each shaped charge in the cluster, which in turn decreases penetration of the formation and reduces to a significant degree the advantages of the clustered charges and large number of resulting perforations.
The problems associated with reduction of stand-off have been recognized in U.S. Pat. No. 3,429,384, issued to J. B. Shore on Feb. 25, 1969, the disclosure of which deals with the reduction of stand-off associated with the use of a very large shaped charge in a tubular carrier, and indicates that even a fraction of an inch increase in stand-off can result in as much as a twenty percent increase in penetration depth. The patent to Shore discloses the use of a tubular perforating gun carrier having concave depressions machined in the exterior thereof, the centers of which are subsequently dimpled outward with a forming tool placed in the interior of the carrier. The resulting configuration provides an increase in stand-off equal to PG,4 the depth of the dimple, while the machined depression and resulting thinning of the carrier wall reduces the outward protrusion and thickness of the burr which is formed by the shaped charge adjacent the dimple when it is fired. These latter phenomena make the carrier less likely to stick in the tubing string as it is retrieved. However, the configuration and method of effecting same as disclosed in the patent to Shore possess a number of inherent disadvantages. First, the method of achieving the concavity with dimple therein involves precision machining of the depressions to a predetermined depth and subsequent use of a forming tool, which must be precisely oriented. Second, the reduction of the wall thickness is not practical for carriers having clusters of three, four or even five shaped charges at a single level, due to the unacceptable decrease in compressive strength in the carrier wall, and the possibility of total destruction of the carrier upon firing of the charges, with attendant clogging of the well bore with debris. Furthermore, precise alignment of the shaped charges within the carrier with each dimple is required for maximum effectiveness.